The present invention relates to an optoelectronic reading apparatus especially suited to Fourier or Fraunhaufer transform holography.
Fourier transform holography provides an advantageous means for recording a large amount of information in a small space. Generally, a large number of closely spaced holograms are formed on a sheet or plate and selectively reconstructed through illumination with a coherent reconstruction beam from a laser. The holograms may represent digital or analog information in two dimensional form. The analog information may be, for example, pictures, printed pages or the like.
A scan means is provided to move the sheet relative to the reconstruction beam and a sensor so that the selected or desired hologram is read. However, a major problem has limited the packing density of such holograms heretofore. The problem is that due to the close spacing of the holograms the scan means must operate with such precision as to render the reading apparatus economically unfeasible. More specifically, the sheet must be positioned with microinch precision to ensure that the selected hologram is in the reading position.
Another major problem has existed heretofore which has limited the resolution of Fourier transform hologram readers. Due to variations in the output of the laser which produces the reconstruction beam, the reconstruction efficiency of various holograms, temperature dependence of dark current in optoelectronic sensors and other effects, it has been excessively difficult to obtain high and stable resolving power of fine patterns. In digital applications, the quantization threshold varies in accordance with the parameters described above. If the quantization threshold is too high fine patterns will appear as unresolved dark areas. If the quantization threshold is too low the fine patterns will appear as unresolved light areas. This effect is compounded by the fact that the average image intensity of fine patterns is lower than that of coarse patterns due to the optical transfer function. In analog applications these variations cause incorrect bias and resulting loss of resolution.
In digital applications, it has been proposed to overcome this problem in high signal-to-noise ratio processing to integrate the output of a sensor which scans a predetermined area of the hologram having a pattern consisting of a predetermined number of bits of logically high or low information. This integrated value is compared with a predetermined value and the quantization threshold adjusted until equality is obtained. Although this method has practical application in digital systems, it cannot be used in analog applications.